There are many applications where the surface and/or the shape of a body is to be measured in a contactless manner. In the production of airplane airfoils, for example, with some types of airplane, the shape of the airfoil is measured after individual production steps so as to ascertain possible deviations of the actual shape from a nominal shape. Another alternative application is the contactless measurement of the surface of castings or moldings, for example, so as to be able to discard defectively produced parts.
Frequently, bodies resulted to for the contactless measurement, is the optical measurement of 3-dimensional bodies, wherein particularly frequent use is made of the light-slit method.
In the light-slit method as it is schematically illustrated in FIG. 5, a light line is projected onto an object to be tested. The course of the light line on the surface of the object is recorded at an angle to the projection direction by means of a camera. This course therefore reflects the topography of the surface and may therefore be used for the 3-dimensional measurement of the surface when the object to be measured is moved under the assembly of laser and camera.
FIG. 5 shows a laser 2 as the light projector, a camera 4 and an exemplary simple geometrical object, that is a cuboid 6, as the object to be measured. By means of suitable laser optics, the laser 2 generates a fanned-out beam, which is projected onto the surface of the cuboid 6 at the measurement position 8. The camera 4 observes the light line at the measurement position 8. As the projection direction of the laser 2 and the observation direction of the camera 4 form an angle, the measurement beam, when there is a change in the surface of the cuboid 6, e.g. an elevation on the surface, is detected by the camera 4 in another location on the light sensor of the camera 4 (e.g., a CCD). From knowledge of the angle between laser 2 and camera 4 as well as knowledge of the detection position of the light beam in the camera sensor, the topographical information on the surface of the cuboid 6 may be obtained at the measurement position 8. If the cuboid is passed under the measurement position 8 in a scan direction 10, a 3-dimensional surface profile of the cuboid to be measured may be created.
In the example shown in FIG. 5, it is in principle only possible to measure one single surface of the cuboid 6, that is the one on which the light projection of the laser 2 is visible at the measurement position 8. In the general case of the 3-dimensional measurement of bodies, there is the problem that only a portion of the entire surface is detected by the light line and the camera, with the rest of the surface not being illuminated. If the entire surface of a body is to be measured, several light lines and one or more cameras therefore have to be used.
FIG. 6 illustrates this by means of a 3-dimensional measurement of a cuboid 20, which is illuminated by a first laser 22 and a second laser 24, wherein, for the sake of clarity, only one camera 26 is shown in FIG. 6. As can be seen in FIG. 6, the laser 1 illuminates the left-hand side and a portion of the surface of the cuboid 20, whereas the second laser 24 illuminates the right-hand side and a portion of the surface of the cuboid.
It is to be noted that for complete measurement of the cuboid 20, several cameras are necessitated; however, the additionally necessitated cameras are not shown as they are not indispensable for understanding the method. For completely measuring the surface of the cuboid 20 by means of camera 1, those proportions of the light lines of the laser 22 and the laser 24 projecting a line onto the surface of the cuboid 20 are to exhibit spatial overlap so as to be able to completely detect the surface. If the lines do not exhibit any overlap, the initial and end points of the different light lines would have to exactly lie on top of each other. However, as the lines are generated by means of a spot laser using special line optics, the length of the light line generated on the object depends on the distance of the laser to the object. In 3-dimensional bodies, the distance of the surface from the various lasers, however, changes from measurement location to measurement location (that is along the scan direction 30). It is therefore not possible to align the initial and end points of the lasers 22 and 24.
Special light-slit sensors in the camera 26 determine the position of a light line as early as on the sensor itself, as this serves to achieve a maximum measurement rate, which amounts to up to 20,000 evaluated light lines per second in currently available sensors.
When using several light lines from different lasers, as it is illustrated in FIG. 6 on the surface of the cuboid 20, however, such sensors are, however, not capable of deciding which of the light lines is to be drawn upon for correct measurement. For bypassing this problem and the associated misinterpretations, the projected light lines would have to seamlessly blend from one to the next. In principle, this may be achieved by having the light lines first aligned in parallel and then having them shifted in parallel until they exactly lie on top of each other. Although this is basically possible, this process involves tremendous adjustment efforts. One additional problem is that this adjustment is not time-stable due to external influences such as the temperature changing or mechanical stresses occurring.
One further problem of the light-slit method is posed by the fact that elevated portions on the surface of the object may cause shadows when using a camera and a laser, which means that portions of the surface may not be detected.
This is illustrated by means of FIG. 7, in which a laser 40, a camera 42 and an object 44 to be measured are schematically shown. The feed direction 46 (scan direction) is symbolized by means of an arrow.
The object 44 to be measured exhibits an elevation 48 so that, when the assembly is used, the given geometry and the rectilinear propagation of light prevent measuring a region 50. In the region 50, projecting a measurement light beam is impossible as same is shaded by the elevation 48.
In principle, this problem may be solved by using two cameras arranged symmetrically relative to the projection direction of the light line, for example. When the laser illuminates the object in a perpendicular manner from above, for example, the cameras will record the position of the light line from two different directions. Obviously, this serves to avoid shading effects as the light beam itself cannot be shaded and at least one camera can observe the light beam in each case. What is very disadvantageous with this approach, however, is the greatly increased cost expenditure caused by the use of a second, complex camera. As the positions of laser and camera are basically interchangeable, it is also possible to combine one camera and two lasers so as to avoid shadings. Due to the above-mentioned problem of the undistinguishability of the laser lines, however, this is impossible in conventional methods.
In general, in the light-slit method, the projected laser lines are merely observed, that is, the method is based on diffusely scattering the laser light at the location of incidence on the object so that the camera may observe the line of the laser on the object without interferences. In the conventional light-slit method, there may however be additional problems when the surfaces of objects to be measured are partly reflecting so that reflections are created, which in the most unfavorable case are reflected into the optics of the camera, thereby corrupting the camera's image. In such a case, in a portion of the camera image in which only the light line should be seen, additional light patches occur, rendering the evaluation difficult if not impossible. If, for example, diffusely scattering car tires on reflecting aluminum rims are measured, this problem may occur when the portion of the light line incident on the aluminum is reflected in the camera range. In this case, the conventional light-slit method is not able to capture the tire.